1. Field of the Invention
This invention relates to a method of forming a light source by the use of a refractive index distribution type lens and to a light source device.
2. Description of the Prior Art
As a heterogeneous lens, there is known a radial index lens endowed with a heterogeneous refractive index distribution in the diametrical direction of the lens from the optic axis or an axial index lens endowed with a heterogeneous refractive index distribution in the direction of the optic axis. Of these lenses, the radial index lens is widely used and one such lens is known as the Celfoc lens (trade name). Herein, the term "refractive index distribution type lens" refers to this radial index lens.
In recent years, it has been known in the field of optical communications to use a refractive index distribution type lens as a collimater to collimate the light beam from a divergent light source such as a semiconductor laser. In this field of optical communications, it has not been necessary to strictly correct the aberration characteristic of the collimater lens. However, where this light source device is used in an instrument such as an optical head, a digital audio disc or LBP, it must be good with respect to aberrations. The refractive index n(r) of a refractive index distribution type lens at a position spaced by r from the optic axis thereof, with the refractive index on the optic axis being n.sub.0, is represented by EQU n.sup.2 (r)={n.sub.0.sup.2 1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 +h.sub.8 (gr).sup.8 + . . . } (1)
where g is a value which determines the power of the heterogeneous paraxial amount and h.sub.i is a value related to the (i-1)th order aberration, and both of these are constants. Accordingly, in the refractive index distribution type lens, to effectively correct aberrations, the lens must be manufactured with the values up to high order h being controlled. However, it is very difficult to manufacture a lens with the values up to such high order h being controlled. Also, in the refractive index distribution type lens, as shown in FIG. 1 of the accompanying drawings, a light beam 2 is regularly reflected by the outer periphery 1a of the lens and it has often been the case that a ghost image or flare is created by the regularly reflected light beam 3.